Limiting Motion in Exposure

It’s easy to think of the camera as this marvelous machine that can stop time. This is certainly a handy metaphor for general thought about photography. Using a camera you can make a long jumper appear to freeze in midair or make the water spurting from a fountain look like a glass sculpture. But, notice I use the word appear. That’s very intentional. The idea that a camera stops time—that somehow the world around us is a sequence of still moments that can be selectively accessed by pushing a button—well, it’s a nice idea. But it’s akin to using algebra to understand physics. It’ll get you started, but won’t adequately finish the job.

The exposure control on my dad’s Nikon FE / ©2009 James Duncan Davidson

A camera doesn’t stop time. Instead, it captures a period of time. This can be is an extremely small period of time, such a thousandth of a second or even less. It can be a relatively long hundredth of a second. Or, it can be multiple seconds, minutes, or even hours long. No matter how short or long the exposure however, the thing to remember is that an exposure builds up over the entire duration of time the shutter is open. If what you point your camera at stays still for the entire duration, whether a fraction of a second or hours, it will appear frozen in time—assuming of course that your camera also remained still. Otherwise, whether or not your subject appears frozen is a function of how much it moved relative to the sensor or film in your camera.

The mental model I carry around in my head for thinking about this comes from calculus in the form of a limit. Now, don’t run away if you skipped or flunked out of calculus in school. I’m not talking about the complicated arcosine inverse third derivative of negative pi stuff that totally made everyone’s ears (well, at least mine) bleed. I’m just talking about simple limits here. The idea that as one value changes, it can only approach but never equal another value. In this case, as the relative motion between subject and camera decreases, you’ll get an ever increasing “freeze factor,” (a made up non-scientific term). If the relative motion between subject and camera is low enough, your image will appear frozen in time. Otherwise, you’ll see motion in your photograph, which can be either a good or a bad thing depending on what kind of image you are making.

Why the focus on relative motion between subject and the camera’s sensor or film? Well, let me show you an image that explains it better than words can. Here’s a photograph I took out the window of a car driving down the road at 70MPH.

Interstate 5 South of Redding, California / Exposure time: 1/45 second / ©2006 James Duncan Davidson

The vehicle’s speed was constant (well, as close to constant as cruise control could hold it) for the 1/45th of a second that the shutter was open on the camera. First, I want you to look at the blurred ground in the highway’s median. It’s blurred because each blade of grass was moving across a significant portion of my camera sensor’s surface. The combination of all that grass moving across the camera frame merged into the result you see. Now, look a bit further out to the scrub on the other side of the road. It’s blurred, but not as much. And, as you look out into the field, the amount of blur decreases with distance. And further out, the buildings are barely blurred at all in comparison.

Now, look at the clouds. Compared to the median and field, they’re pretty sharp. This is because they are so far away that they moved across a very small amount of my camera’s sensor. In fact, I doubt my speed down the highway contributed at all to blurring them. Instead, I’m pretty sure that camera shake and my car’s window are probably responsible for whatever blur is apparent in the clouds. There was quite a bit of natural vibration in the car and I was hand holding the camera. (Important disclaimer: I don’t recommend or endorse taking photos while driving!)

You can never eliminate every single last bit of motion between your subject and camera. The entire universe is in a state of constant motion at some level. That’s why treating this as a limit function makes sense. Like many other phenomena that can be modeled with limits, however, absolute perfection doesn’t matter. What matters—at least if you’re not wanting motion blur in your photographs—is that you reduce the relative motion between camera and subject enough to make so that the result appears crisp.

Waves in Boiler Bay, Oregon / Exposure time: 1/500 second / ©2008 James Duncan Davidson / Order a Print

How much is enough? Well, it depends on a lot of factors. But ultimately, if you want to milk out the most that your camera’s sensor and lens can provide, you’ll need to get the relative motion of subject on your camera’s sensor or film to be less than what your camera can resolve. In digital terms, you don’t want the image projected by your lens moving more than some percentage of the width of pixels in your camera’s sensor. When you consider that pixel sizes of less than 10µm are common, that’s not a lot.

As I’ve mentioned, there are two sources of motion you have to think about. The first camera shake, the motion of the camera’s sensor or film relative to the scene. The second is the motion of the subject relative to your camera. To limit camera shake, you’ll want to make sure your camera is well supported.

If you want the crispest of landscape or architecture shots, you’ll want to use a tripod. And mirror lock up. Every. Single. Time. As you’d expect from human nature, you’ll more photographers saying this than actually doing it. But, quite honestly, when I’m shooting out in the landscape I either a) use a tripod; b) use some other support such as a handy fence pole or tree; or c) kick myself for not having my tripod because I know that I’m not doing my part of the job and the quality of the photographs I’m making will suffer for it. In the interest of full disclosure as to my own adherence to my own advice, three out of the four photographs in this post were made with a tripod. Even the relatively fast 1/500th exposure of the waves above was made with a tripod.

If a tripod isn’t practical and you are shooting subjects a bit less critical of crispness, such as people, then you’ll need to make sure you’re using good technique in handholding your camera. You’ll also want to make sure to use a fast enough shutter speed to minimize any resulting motion on your part. The conventional rule of thumb to use is 1/(focal length). In other words if you’re using a 50mm lens, keep your exposure at 1/50th or faster. Some photographers can hold their gear quite steady and can get away with lower speeds. Others will need to use to a higher speed. Some experimentation to find out how stable you can shoot isn’t a bad idea. Image stabilization, such as Canon’s IS or Nikon’s VR systems help quite a bit with this as well. Even so, with a longer lens and lower light conditions, some sort of support can be really handy, even if it’s just using a wall or a handy railing.

Placa Catalunya Fountain / Exposure time: 0.6 seconds / ©2007 James Duncan Davidson

As far as motion on the part of your subject, the exposure time needed will vary greatly. A building or a landscape is typically quite patient and will sit still all day long. If you’ve got your tripod out, use whatever exposure time you need to. On the other hand, people don’t stay very still. You’ll probably want to use at least the 1/(focal length) rule and maybe two or three times faster than that. In my own work, I try to shoot people at least at a 1/125th if they are moving at all, unless I want to emphasize that movement. If you’re taking photos of Formula One cars, on the other hand, you may need to go as high as your camera can shoot to limit motion blur, assuming you have enough light.

Finally, remember that getting a crisp shot, or a shot filled with lots of beautiful motion blur, is a matter of playing the limit function. Make some test photographs and examine the results. If you need a crisper shot, go with a faster exposure time. If you want more motion blur, go with a lower exposure time. With time and practice, you’ll be able to dial in a starting point for the situations you find yourself in most and work things from there.

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